(273b) A Molecular Look at Single Ion Conductors for Battery Application: Interplay of Free Ion Content, Polymer Mobility, and Conductivity

Solid state polymer electrolytes (SPE) offer several advantages over liquid electrolytes for use in lithium ion batteries. Despite the advantages, a major obstacle for practical use of solid state electrolytes is their low conductivity in room temperature. A complicating feature in SPEs is the contribution of both the cation and anion to conductivity. To isolate the cation contribution, we study a single ion conductor, in which the anion is covalently bonded to the polymer backbone thereby rendering it immobile. The material is an ionomer with a small number of charged monomers co-polymerized with PEO, the best candidate for SPEs due to its ability to solvate lithium and other alkali cations. We use molecular dynamics simulation to observe the interplay of cation-anion association, polymer mobility and cation motion. Cation-anion association controls the number of free ions, i.e. those that are able to contribute to conductivity. Polymer mobility controls how fast the free ions are able to move through the SPE. High conductivity requires both a high free ion content and fast polymer motion. To understand the connection between the two, we ?tune? the force field in order to manipulate the free ion content and observe the influence on PEO dynamics. The results show that high free ion content and fast polymer mobility are mutually exclusive: as the free ion content rises, more cations associate with the polymer chains, reducing their mobility. This suggests that there is a trade off between free ion content and PEO mobility. Movement of the cation is greatest at intermediate free ion concentration, and attempts to increase the free ion content or increase polymer motion both lead to decreased conductivity.